Many types of machines have a control in the form of a mechanical device, such as a lever, to operate the machine between a stopped condition, perhaps an idle condition, to a fully engaged position wherein the machine is operating at maximum capacity and oftentimes the control or lever will have intermediate settings or positions between its stopped or rest position and its position of maximum movement so as to operate the machine at an intermediate level of operation. One example of such a machine is a trencher which moves along the ground and progressively produces a trench in the ground for the purpose of laying a pipe or wire, or some other similar type of facility. The trencher has a control which regulates the travel of the machine along the ground; and the trencher also has another control which regulates the trench digging facility of the machine. It may be desirable to operate these controls for the trencher from a position remote from the actual levers or physical controls which open the valves or operate electrical devices.
Another such example is a concrete mixing truck where it is desirable to be able to control the discharge of the flowing concrete being carried by the truck from a position within the cab of the truck. In this instance, it may be desirable to operate a hydrostatic pump from a position remote from the pump, in order to cause the concrete mixing to either discharge the concrete or discontinue the discharging of concrete. Of course, operating the valve may be by a push-pull device or by a swingable or tiltable lever.
Linear actuators for performing such control functions have been known previously, and have utilized an electric motor, the speed of which is geared down to turn a low pitch screw to operate a nut and cause travel of the nut which is connected to a push-pull rod for operating a cable to control the machine. Such linear actuators have been successful in many respects, but have had certain significant problems.
Where such previously known linear actuators have been used, oftentimes they are used in situations such as construction sites where many facilities are being simultaneously used and the machines and the controls are subject to damage. If the electrical power being supplied to such a previously known linear actuator is suddenly terminated, the electric motor will suddenly stop, or if the nut and push-pull rod has already been moved and extended, the push-pull rod or the cable will simply stay in its extended position and of course, the machine being controlled will continue to operate at the conditions to which it has been previously set. If the machine is a trencher, it may be that the trencher will continue to travel along the ground, out of control, or the trenching device may continue to dig deeper and deeper, depending upon which controls are being regulated.
In the case of a cement truck, if the power to the actuator motor is suddenly terminated, it may be that the concrete mixing truck will continue to discharge excessive amounts of concrete where only a small amount of concrete is desired.